Abrasive jet machining may be defined as the removal of material through the reaction of a focused, high velocity stream of fine grit or powder-loaded air. Basic apparatus for carrying out the abrasive jet machining process may include an air or gas supply, a pressurized powder reservoir with cooperating vibrating means (or a non-pressurized powder reservoir using venturi principles), a mixing chamber, and a conduit for feeding the resultant powder to a hand held piece having an appropriate nozzle for directing the powder-air mixture against a workpiece for delicate removal of portions thereof. A foot or remote switch may control the air pressure at the nozzle. The pulsation of the vibrating means or the positive pressure within the powder reservoir urges the powder through a narrow orifice or passageway to contact a pressurized air stream or jet. The amount of powder delivered is dependent upon the amplitude of vibration, pressure of the delivery system, diameters of the handpiece nozzle and power feed supply orifice or passageway, and size of powder, among other factors.
The abrasive powder, such as aluminum oxide and silicon carbide, for example, should be well classified and clean, and typically ranges in size from about 10 to 150 microns in diameter.
Abrasive jet machining is not a mass material removal process, but one of finishing or precision cutting. Because of the small amount of abrasive powder flowing through the handpiece nozzle at any given instant, it is not difficult to remove selected portions of the workpiece, such, for example, as the removal of conformal coatings from printed circuit boards. During removal of the coatings, static electric charges of several thousand volts readily build up on the workpiece in a matter of only several seconds. Several prior art devices have been developed which attempt to reduce or eliminate these undesirable charges by introducing a mist or jet of water at or adjacent the work site. These devices have indeed reduced charge build-up to levels as low as 500-700 volts, which voltages however, are still sufficiently high to produce discharges which often damage the delicate circuitry of modern printed circuit boards.
The present invention provides abrasive jet machining apparatus which produces no measurable static electric build-up on the workpiece, and hence, is suitable for use with the most delicate of printed circuits.
Briefly, the present apparatus employs a conducting solution of an electrolyte, typically NaCl, although not limited thereto, as the liquid or mist medium. The solution however, unlike prior art devices, must be electrically grounded in order to instantaneously conduct any charge build-up on the workpiece away therefrom to ground. The solution may be contained in a grounded metallic container or receptacle, or the receptacle may comprise a non-conducting material such as plastic, for example, having a grounded metallic article immersed within the solution. Alternatively, a wholly ungrounded plastic solution container may be used if the solution within the tubing leading to the handpiece is suitably grounded, or the metallic nozzle itself, if such a nozzle is employed through which the solution must pass, is grounded. We have found that static build-up on the workpiece may even be prevented by interposing a grounded wire in the path of the jet stream of solution or mist between the solution nozzle and workpiece.